Recent perceptual research distinguishes between stimulus dimensions which are perceived continuously and those characterized by categorical perception. The perception of many continua in speech (and some in music) is said to be categorical: stimuli on such continua can be discriminated only to the extent that they have different labels. For dimensions which are perceived continuously, discrimination exceeds that predicted from labeling data. The categorical-continuous distinction is based, however, on the highly restrictive psychophysical assumptions which underlie threshold theories of perception. A Signal Detection Theory (SDT) analysis of the discrimination designs common to experiments on categorical perception can yield unbiased sensitivity measures; threshold analysis cannot. The SDT analysis also leads to a different predicted relation between discrimination and labeling when perception is categorical. Finally, the model suggests an alternative explanation for data often cited in support of the hypothesis that some sounds are subjected to dual processing along categorical and non-categorical continua; it is possible to distinguish these hypotheses experimentally. The proposed research tests this SDT model of categorical perception, using speech stimuli, musical stimuli, and such simple stimuli as tones differing in frequency. Discrimination experiments will be performed to evaluate the success of the SDT and threshold models in relating different discrimination paradigms. Experiments incorporating both discrimination and labeling tasks will then be used to determine whether a particular auditory dimension is perceived categorically, non-categorically, or along both categorical and non-categorical dimensions. The categorical perception hypothesis has had a strong influence on the theories of speech perception that underlie much applied work in automatic speech recognition and production. It is important to provide testable models for the hypothesis that avoid the flaws of the models now in use.